Gallien Krueger 700rb Protection Mode

[buffalo]

I recently bought a GK 700rb amp. The previous owner said the place he played had bad electrical issues. I replaced the fuse (the old one was not present, but there was black on one of the fuse terminals on the underside of the PCB). The Amp turns on, but stays in protection mode. No sound comes through. All the components visibly look alright. How could/should I go about trouble shooting this amp?

[loudthud]

First thing is get/build a light bulb limiter. I don't know the particulars of the 700RB but check for a big DC voltage on the output. There are probably one or more shorted output or driver transistors that can be found with an ohm meter. Also, ohm any "sand box" resistors as these usually go when a transistor shorts. If any of this is over your head and you are not familiar with sourcing and replacing transistors, I suggest you take the amp to a tech for repair.

[buffalo]

ok, thanks. I will start there.

[Enzo]

Protection mode usually means there is DC on the output, but there is either a speaker relay that stays off or the power supply shuts down. Without me looking it up I'd bet on the former. Is there a relay in there? Not tiny one, but one like an inch square or larger.

Look at the output transistors. are there also some cement power resistors with fractional ohm resistance - like .33 ohm or .22 ohm or something nearby? With power on, is there large DC to ground from any of these resistors?

With power off, are any of the power transistors shorted?